The long term objective is to understand the eukaryotic nucleus, using the yeast, Saccharomyces cerevisiae, as a model system. The goal is to identify and characterize genes affecting the structure and function of chromosomes. Current effort is focused on understanding the mechanism of transcriptional silencing of the yeast silent mating-type loci and telomeres. These silenced chromosomal domains are the yeast equivalent of heterochromatin in metazoans. The roles of Sir3 and Sir4 proteins in silencing will be studied in detail. Specifically, the role of the Sir3 N-terminal BAH domain and a conserved domain of Sir4 will be investigated. The possible role of acetyl ADP-ribose as a signalling molecule in silencing will be examined. The possible sumoylation of Sir4 will also be studied. Another aim will be to search for a histone demethylase gene. Recent work has shown that methylation of specific lysine residues of histones H3 and H4 is very important for gene regulation. There is some evidence that this modification is reversible but no lysine demethylase gene or enzyme has been identified. Various biochemical and genetic approaches will be undertaken to find such a gene. Progress in these areas will shed light on chromosome structure and function in the eukaryotic nucleus, not only in yeast but also in mammalian cells.